One of the most popular methods for measuring the displacement caused by oscillation of a micro cantilever is the optical beam deflection method, which has been widely used in commercialized atomic force microscopes (AFM). Since the optical beam deflection method lacks direct traceability for displacement variation, and a comparison calibration method is needed for converting deflection into unit of length, namely meter. Besides, the optical beam deflection method cannot optimize both the detection sensitivity and the spot size of detection light at the same time. That is, the smaller the spot size, the larger the magnification factor of the focusing lens, and the worse the detection sensitivity.